It is not unusual for a puppy to have an underbite ( or as we usually say here in the UK an undershot mouth)
If slightly undershot at 6 months it may well come right. I had an undershot puppy that eventually ended with his bottom teeth just level with the top teeth when he was eighteen months old.
He was a very nice dog & I showed him at Championship shows. He sometimes won his class ( and he was awarded a reserve CC ) Other times he got thrown out.
Judging is all about weighing up the faults and virtues of the individual dogs. How well we did on any one day very much depended on whether that particular judge felt a completely correct bite was more important than good conformation & movement.
Cavaliers......Faith, The Ginger Tank and Woody.
Japanese Chins.... Dandy, Benny, Bridgette and Hana.
Remembered with love......... Tommy Tuppence and Fonzi